beta-Crystallins (beta 1-, beta 2- and beta 3-
crystallin) comprise nearly half the
protein of the human lens. The effect of near-UV radiation, which is one of the possible risk factors in
cataract formation, on the
beta-crystallins is investigated in this study.
Protein intersubunit crosslinking, change in charge of the
protein subunits to more acidic species and changes in
protein tertiary structure (conformation) by 300 nm irradiation are reported. The fluorescence yield of
protein tryptophan residues decreases by 300 nm irradiation. There is an increase in nontryptophan fluorescence (lambda cx 340 nm, lambda cm 400-600 nm), and in
protein absorption at 340 nm, due to the formation of
tryptophan photooxidation products. Both
tryptophan and its oxidation products can be photoexcited by 300 nm irradiation and the latter are known to be good
photosensitizers. The results provide evidence for the generation of H2O2 in the irradiated human
beta-crystallin solutions by the Type I photosensitizing action of the chromophores absorbing at 300 nm. The H2O2 is generated via the intermediate production of O2
anion; the latter spontaneously dismutates to H2O2, presumably via O2-
protein interactions. The amount of H2O2 generated per absorbed photon is compared for various solutions of beta 1-, beta 2- and beta 3-crystallins from human
lenses of different age.